At present, the Institute of Electrical and Electronics Engineers (IEEE) 802.16j standardization group is in the process of standardization work for a Mobile Multihop Relay (MMR) technology. Advanced research institutes and enterprises of various global countries conducting research on 4th-Generation (4G) mobile communication are competitively promoting the development of a wireless multihop transmission technology.
Basically, a relay station (RS) demodulates and decodes data received from a transmitter, and then again encodes and modulates the data for transmission to a receiver. The RS enables high-speed data communication by reducing a path loss between wireless links, and enables communication even when the receiver is far away from the transmitter or is located in a shadow area.
In a conventional RS technology, in general, an RS performs transmission and/or reception using different time slots or different frequency resources. That is, the RS operates in a half-duplex. This is because a strong transmitted signal is added to a weak received signal, and the transmitted signal and the received signal interfere with one another if a transmit antenna and receive antenna of the RS are insufficiently isolated from each other. Because of such limitations, an RS may cause a waste of limited wireless resources and thus a deterioration of a system capacity. Also, in order to apply the RS technology to a conventional wireless communication system not taking an RS into consideration, it is required to change standards (such as a frame structure and so forth) for separation of transmission and reception resources.
A method for overcoming general limitations in the conventional RS technology is a full-duplex relay method for enabling transmission and reception by the same resources (e.g., time slot, frequency, and so forth) using interference cancellation. The full-duplex relay method is a method for enabling transmission and reception with the same resources by enabling an RS modulator/demodulator (modem) to estimate and cancel interference of a transmitted signal on a received signal using a digital signal processing technology. A full-duplex RS enables transmission and reception using the same resources. This increases an available resource of an RS, the flexibility of resource allocation, and system capacity. Also, because there is no need for resource separation for reception and transmission, the full-duplex RS can provide a relay service without a standard change of an existing system or through a minimum change thereof.
However, the full-duplex RS requires an interference cancellation technology for canceling interference caused by a transmitted signal of a transmitter in a receiver, and the interference cancellation basically requires estimating an interference channel of the transmitter. Also, because of interference of the transmitter, the full-duplex RS has to perform a process of synchronization considering interference with an existing mobile station (MS). That is, when the full-duplex RS intends to perform synchronization, if interference caused by a transmitted signal of a transmitter is basically generated because of a characteristic of the full-duplex method and thus a conventional synchronization process is performed, synchronization performance is deteriorated by not considering the interference. Therefore, the full-duplex method requires a process of synchronization considering the interference caused by the transmitted signal of the transmitter.
If a full-duplex RS is a Single Input Single Output (SISO) system using a single antenna for a transmitter and receiver, the full-duplex RS can estimate an interference channel of a transmitter using all transmitted signals as pilots. However, if the transmitter/receiver uses Multiple Input Multiple Output (MIMO) using a multiple antenna, orthogonality between transmit signals transmitted from transmit antennas is not kept, thus causing inter-antenna interference and deteriorating performance in estimating an interference channel. Therefore, in an environment with multiple antennas, the interference between antennas must be considered when estimating an interference channel of the transmitter.